function start() {
  let box = document.getElementsByClassName('box')[0]
  let i = 0
  let id = setInterval(function () {
    i++
    box.innerHTML = i
    if (i === 10) {
      // 清空定时器
      clearInterval(id)
      i = 0
      console.log('定时器被清空了')
    }
  }, 1000)
}

// 点击开始从10--->1
